To elaborate on Old-Biscuit's response IF this is a check valve problem in the water pump:
There may be a check valve inside the water pump that keeps City Water from flowing backward through the pump and filling the fresh water tank. Sometimes you can clear a check valve that is stuck open by:
- Turn City Water off.
- Turn the water pump on.
- Open a faucet full blast for a few seconds and then close it. Repeat a few times.
- Turn the water pump off.
- Turn City Water back on.
If you're lucky the sudden rush of water through the pump will unstick that check valve or clear whatever is holding it open.
If not, many people simply install a check valve on the water pump intake to take the place of the internal one instead of buying a new pump. This one is reported to work well but I've never needed it, yet.
https://www.amazon.com/gp/product/B0...KIKX0DER&psc=1
If you switched to running solely off the water pump to use up the water in the fresh tank and you hear the water pump cycling on and off that will confirm a stuck check valve (assuming there is no leak inside).
With a stuck check valve the water pump will cycle on and off. As the pressure in the RV water lines decreases because water is slowly leaking backward into the fresh tank the pump will turn on to bring the pressure back up.
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A workaround is available if you have a winterization valve on the water pump inlet.
The winterization valve is on the inlet side of the water pump. When the valve is in the usual position the valve allows the water pump to suck water out of the fresh water tank. When the valve is turned to the other position, the water pump inlet is switched from the fresh tank to a short piece of water hose.
When winterizing, you remove the cap from the end of the short hose and place it in a jug of RV antifreeze. The water pump sucks the antifreeze out of the jug and into the water system.
The workaround: Because the winterization valve disconnects the water pump from the fresh tank, you can temporarily move the winterization valve to the winterization position AND assure the cap is on tight.
When City Water tries to flow backwards through the pump it cannot go into the fresh tank; it goes into that short piece of capped-off hose. Of course, if you need to use the water pump you need to remember to change the winterization valve back to the normal position.
Make sense?
Ray